Fix user manager window's rounded corners by ensuring its content view is layer backed.

chrome/browser/ui/cocoa/profiles/user_manager_mac.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/cocoa/profiles/user_manager_mac.h"
 
 #include "base/callback.h"
 #include "base/mac/foundation_util.h"
 #include "chrome/app/chrome_command_ids.h"
 #import "chrome/browser/app_controller_mac.h"
@@ skipping 298 matching lines @@
   [window setMinSize:NSMakeSize(UserManager::kWindowWidth,
                                 UserManager::kWindowHeight)];
 
   if ((self = [super initWithWindow:window])) {
     userManagerObserver_ = userManagerObserver;
 
     // Initialize the web view.
     webContents_.reset(content::WebContents::Create(
         content::WebContents::CreateParams(profile)));
     window.contentView = webContents_->GetNativeView();
+
+    // When a window has layer-backed subviews, its contentView must be
+    // layer-backed or it won't mask the subviews to its rounded corners. See
+    // https://crbug.com/620049
+    // The static_cast is just needed for the 10.10 SDK. It can be removed when
+    // we move to a newer one.
+    static_cast<NSView*>(window.contentView).wantsLayer = YES;
+
     webContentsDelegate_.reset(new UserManagerWebContentsDelegate());
     webContents_->SetDelegate(webContentsDelegate_.get());
 
     web_modal::WebContentsModalDialogManager::CreateForWebContents(
         webContents_.get());
     modal_manager_delegate_.reset(
         new UserManagerModalManagerDelegate([[self window] contentView]));
     web_modal::WebContentsModalDialogManager::FromWebContents(
         webContents_.get())->SetDelegate(modal_manager_delegate_.get());
 
@@ skipping 251 matching lines @@
   [window_controller_ displayErrorMessage];
 }
 
 void UserManagerMac::SetSigninProfilePath(const base::FilePath& profile_path) {
   signin_profile_path_ = profile_path;
 }
 
 base::FilePath UserManagerMac::GetSigninProfilePath() {
   return signin_profile_path_;
 }